The present invention relates to a device for changing and measuring the polarisation of radio waves and to a sensor which measures mechanical angles of rotation or shifts via the polarisation of radio waves. It shall be possible to take measurements quickly and without great latency (in real time), in order to allow the device to be used in control loops for fast-moving objects such as in servos.
Using a suitable sensor a mechanical angle of rotation can be measured by measuring polarised light (DE 10 2005 031 966 B4). An angle sensor of this kind has a number of positive properties, which distinguishes it from other angle sensors. The most obvious property is the translation invariance between sensor and signal transmitter. This on the one hand, leads to distinct simplifications during adjustment and calibration and on the other, to insensitivity in relation to mechanical vibrations.
In certain situations angle measuring with polarised light, despite its robustness, still suffers from some disadvantages. One disadvantage is the use of optical materials as transducers. Some of them are brittle (glass) or scratch-sensitive (plastic foils). On the sensor side it is difficult to integrate transmitters and receivers of light, since both are typically manufactured from different materials, although some progress has already been made (e.g. OLED on chip). Besides the lifespan of LEDs is limited, especially for high temperatures or a corrosive environment. Finally the use of light requires transparent and more or less clean surfaces, which on the one hand restricts, what materials can be used, and on the other, in extremely rough conditions, makes operation of the sensor difficult.
It would therefore be nice if the advantages of polarisation measuring could be upheld, whilst eliminating the problems connected with optics, and if the temperature range, which is limited due to the LEDs and optical polarising filters, could be widened.
Polarisation is an important property of all electromagnetic waves, from radio frequencies to optical frequencies and beyond. In the field of optics polarisation is used in the most varied situations, such as for the suppression of reflections or the visualisation of mechanical stresses. Surface characteristics too can be examined with the aid of a polarisation sensor (EP 1 507 137 B1). Polarisation of light is also used for modulating optical signals in communications technology (CA 2 193 754C).
In the area of radio waves (referring to radio frequencies in the widest sense) polarisation is used sometimes in order to increase the data rate of a communication channel because orthogonally polarised waves do not interfere with each other (GB 618 615 A). Conversely the mostly polarised energy emitted by antennas can lead to undesirable effects, for which, for example, bad reception is an indication in the case of a badly aligned radio antenna.
DE 10 2011 078 418 A1 describes an ellipsometric analysis of surfaces by means of millimeter waves which is also based on the evaluation of a change in polarisation. This relates to the determination of roughness, layer thicknesses and other parameters.
When changing over from the wavelengths of light to distinctly slower radio waves, the materials for these waves change considerably. There is also a marked change in the components for transmitter and receivers and in the properties of transmitters and receivers. As such an antenna for radio waves has nothing in common with an LED or a photodiode. As regards the materials, the considerably longer wavelengths of radio waves lead to the effect that dirt is much less influential which for a large part, is due to the relationship between wavelength and object size, but also to the properties of the dirt. Whilst metal and sand have comparable properties for optical frequencies, the effect upon longer radio waves is completely different.
In radar technology too, polarisation is used in some cases, for example in order to measure the position of an object such as a hidden pipeline (U.S. Pat. No. 4,728,897). Here use is made of the fact that a weak echo of a hidden elongated object can be distinguished from highly undesirable echoes if one repeats a radar measurement with different polarised transmit signals and forms correlations. However, radar technology as a rule relates to measuring distances and speeds, e.g. measuring the runtime or the Doppler Effect. Polarisation in this context is rather disruptive since it can aggravate the detectability of certain objects because the radar echo of an object can change with its alignment with respect to the radar unit.
Radar is typically applied to the detection of an unknown object/an object not belonging to the radar unit. This may be a hidden pipe, another ship or an aeroplane, a car or a human being.
The radar frequencies used depend on the respective application because e.g. achievable ranges and resolutions change depending on the frequency and because the object which is expected to be detected (car or human being) has to be reliably detected. Here, very high frequencies could, for a long time, only be generated by means of special tubes, later via expensive special semiconductors, but latterly also in silicon. Apart from the extraordinary advances in microelectronics which make it possible to use conventional switching technology as far as into the lower THz range, there are very skilful approaches to operate semiconductors in the way tubes used to be operated, such as by stimulating plasma vibrations (Öjefors, Pfeiffer, “A 0.65 THz Focal-Plane Array in a Quarter-Micron CMOS Process Technology”, IEEE JSSC Vol. 44, No. 7, July 2009). Such approaches permit the generation and detection of radio waves in the THz range with comparably small and low-cost systems and, in the ideal case, can be monolithically integrated.
The determination of angles, for classic radar units, means an angle in space to an object, which is determined either via a rotating antenna array or via triangulation (DE 10 2008 019 621 A1). An angle of rotation in relation to a previously determined object can, in certain cases, be determined by approximation via the comparison of echoes of varying size with different polarisation. This presumes that the object to be measured is anisotropic even for the radar frequency/wavelength. That means that the object to be measured has to have anisotropic structures, which bear a certain relation to the wavelength used. A pronounced anisotropy results if the structures are smaller than the wavelength, but not so small as to prevent an interaction. The situation is aggravated in that, as a rule, the object to be measured has a random 3D alignment in relation to the radar unit thereby making an accurate measurement of angles of rotation impossible. In the case of looking for a pipe, angle measurement can under certain circumstances be carried out relatively accurately if the radar unit is held in parallel to the pipe. In this case the radar unit is moved during measuring and possibly rotated until an accurate localisation and orientation is found, whilst the object is stationary.
It is not a matter of course that the radar unit can measure the spatial orientation of an object. For very extensive objects and high-resolution radar units an image of the object, and therefrom its orientation, can be ascertained. For a stationary unit this is normally not successful. In this case polarisation may be helpful in certain circumstances:
For a 10 GHz radar which operates at wavelengths of 30 cm, a grid consisting of 1 cm thick rods and a few 10 cm long rods is a good polariser which allows the conclusion that the grid is rotated transversely to the radar beam. The wing of an aircraft on the other hand, would not generate such pronounced polarisation information because a large part of the reflection comes from its massive surface. Conversely an extremely fine optical polarising filter does not affect this radar radiation because the tiny, partly molecular structures show hardly any interaction with this frequency. The fact that a wing or a pipe generates strong and partially polarised echoes as a function of the polarisation of the incident rays, is due to effects, which are similar to those of the reflection of light on glass (see Brewster angle for complete polarisation of reflected waves) as well as to lens-type effects through surface-induced currents, wherein the surface structures, in conjunction with the direction of the current flow (polarisation direction), have an influence on the “lens characteristic” of the object. Such structures therefore, due to the macroscopic shape and structure of their surface, comprise polarisation-dependent characteristics, which vary however, depending on the irradiated location and the angle of incidence of the radio waves. Therefore accurate angle measurements on such structures with the aid of polarisation are possible only with the help of reference measurements and an accurately adjusted position between radar and object.
Apart from the structure size of polarisation-changing structures account has to be taken, with radar measurements, of the minimum distance to the object to be detected. If the distance is too small, the transmit signal and the receive signal can hardly be distinguished from one another. Further there are near-range effects which can be called proximity effects and which are due, less to wave propagation than to the existence of quasi-static fields. For example, a reflector which is too close to the antenna, may have an effect which is more akin to a capacitive electrode. Echo signals in the widest sense are determined both through capacitive and inductive couplings with the reflector and through runtime and waves, but also through resonance effects and the tuning quality of the badly defined resonator (both radar and reflector reflect a part of the signal, the distance between both of them determines the possible resonance frequencies. Stationary waves can falsify to a large extent the desired signal, in particular because stimulation close to the resonance frequency leads to big phase shifts). A transducer should therefore preferably not lie in the near-field of the antennas.
The measuring of angles of rotation of, say, an antenna mast with quasi-static signals is described e.g. in GB 1 493 988 A, where two phase-shifted transmit signals are generated, and by mixing them with the receive signal, a mixed signal is generated which comprises two frequency-shifted components depending on the rotational frequency, and the electrical phase position of which, in relation to the transmit signals, is dependent on the angle of rotation of the mast. This embodiment is not suitable for determining the angle of rotation of the stationary or slowly rotating unit, because here the two frequency components merge with one another.
The DE 198 130 41 A1 describes a device for measuring rotating objects, which is based on the Doppler Effect and which measures a spectrum of the echo and compares it with a reference spectrum, in order to detect the wear of a tool and other error conditions. This achievedhod is however, sensitive to vibrations and not suitable for ascertaining the angle of rotation of a stationary object.
The DE 101 42 449 A1/DE 101 32 685 A1 describes a method for determining an angle of rotation or a distance, which is based on measuring the signal phase (of a delay). Again, with this achievedhod evaluation of the signals is aggravated due to vibrations because a change in position of the transducer leads to a change of the signal phase, which is not easy to distinguish from a rotation of the object.